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We report the first detection of C$^{15}$N in diffuse molecular gas from adetailed examination of CN absorption lines in archival VLT/UVES spectra ofstars probing local diffuse clouds. Absorption from the C$^{15}$N isotopologueis confidently detected (at $\gtrsim4\sigma$) in three out of the fourdirections studied and appears as a very weak feature between the main$^{12}$CN and $^{13}$CN absorption components. Column densities for each CNisotopologue are determined through profile fitting, after accounting for weakadditional line-of-sight components of $^{12}$CN, which are seen in theabsorption profiles of CH and CH$^+$ as well. The weighted mean value ofC$^{14}$N/C$^{15}$N for the three sight lines with detections of C$^{15}$N is$274\pm18$. Since the diffuse molecular clouds toward our target stars haverelatively high gas kinetic temperatures and relatively low visual extinctions,their C$^{14}$N/C$^{15}$N ratios should not be affected by chemicalfractionation. The mean C$^{14}$N/C$^{15}$N ratio that we obtain shouldtherefore be representative of the ambient $^{14}$N/$^{15}$N ratio in the localinterstellar medium. Indeed, our mean value agrees well with that derived frommillimeter-wave observations of CN, HCN, and HNC in local molecular clouds.

The C$^{14}$N/C$^{15}$N Ratio in Diffuse Molecular Clouds